GNSS reflectometry

Remote sensing allows the retrieval of geophysical parameters of interest, such as sea surface salinity, soil moisture, sea state or altimetry products. On the other hand, GNSS (Global Navigation Satellite Systems) cover the Earth with their navigation signals, used nowadays in a wide range of everyday situations, such as fleet management, vehicle guidance or leisure/outdoors interactive maps. However, as first proposed more than twenty years ago, these navigation signals can also be received and processed after reflecting on a certain surface to retrieve altimetry information. This approach, in short known as GNSS-R (Global Navigation Satellite Systems Reflectometry) [1,2], allows for inexpensive remotely sensing of geophysical parameters over wide areas of the Earth’s surface. This is possible because the scattering mechanism ‘watermarks’ the signal so that the geophysical information regarding the reflecting surface is added to it. A very straightforward example is that of the soil moisture retrieval. The water content is related to the soil dielectric constant, which is related to the reflection coefficient. Therefore, the ratio between the incident and the reflected power of the GNSS signal can be indirectly linked to the soil moisture content. In addition, the emitted GPS signal is RHCP (Right-Hand Circularly Polarized). However, after reflecting on the surface, it becomes mostly LHCP (Left Hand Circularly Polarized), although with a certain degree of ellipticity due to the difference between the vertical and horizontal reflection coefficients. All this wealth of information can be used to infer geophysical parameters of the ocean [3], land [4,5], and ice surfaces, such as: